On the hydrodynamics of soap films

Several experiments aiming at the exploration of the hydrodynamical properties of soap films are presented. Their interpretation takes into account the very specific equation of state of these films. It is shown that on short time scales each element of the film moves as a whole so that the film can be considered as a two-dimensional fluid with a local density proportional to its thickness. When set horizontally, quasi two-dimensional turbulent flows can be obtained. The film behaves as an incompressible fluid whenever the motions occur at velocities small compared to the velocity of its elastic waves. An estimate of the role of air friction is given. The static quasi equilibrium of a film when set vertical is discussed. Phenomena equivalent to the rise of buoyant bubbles can be obtained. It is shown that lee waves can also be generated confirming that a vertical soap film has the dynamical properties of a two-dimensional density stratified fluid.